\Header{rowsum}{Give row sums of a matrix or data frame, based on a grouping variable}
\methalias{rowsum.data.frame}{rowsum}
\methalias{rowsum.default}{rowsum}
\keyword{manip}{rowsum}
\begin{Description}\relax
Compute sums across rows of a matrix-like object for each level of a grouping
variable. \code{rowsum} is generic, with methods for matrices and data
frames.\end{Description}
\begin{Usage}
\begin{verbatim}
rowsum(x, group, reorder = TRUE, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] a matrix, data frame or vector of numeric data.  Missing
values are    allowed.
\item[\code{group}] a vector giving the grouping, with one element per row of
\code{x}.  Missing values will be treated as another group and a
warning will be given
\item[\code{reorder}] if \code{TRUE}, then the result will be in order of
\code{sort(unique(group))}, if \code{FALSE}, it will be in the order
that rows were encountered. 
\item[\code{...}] other arguments for future methods
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The default is to reorder the rows to agree with \code{tapply} as in
the example below. Reordering should not add noticeably to the time
except when there are very many distinct values of \code{group} and
\code{x} has few columns.

The original function was written by Terry Therneau, but this is a
new implementation using hashing that is much faster for large matrices.

To add all the rows of a matrix (ie, a single \code{group}) use
\code{\Link{rowSums}}, which should be even faster.\end{Details}
\begin{Value}
a matrix or data frame containing the sums.  There will be one row per
unique value  of \code{group}.\end{Value}
\begin{SeeAlso}\relax
\code{\Link{tapply}}, \code{\Link{aggregate}},\code{\Link{rowSums}}\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
x <- matrix(runif(100), ncol=5)
group <- sample(1:8, 20, TRUE)
xsum <- rowsum(x, group)
## Slower versions
xsum2 <- tapply(x, list(group[row(x)], col(x)), sum)
xsum3<- aggregate(x,list(group),sum)

\end{ExampleCode}
\end{Examples}

